1. Field of the Invention
The present invention relates to the production of high strength carbon foams having a direction of enhanced conductivity, especially thermal conductivity. The foams can be graphitized to provide a graphite foam having a direction of enhanced thermal and electrical conductivity. Methods for the production of such foams are also disclosed.
2. Description of the Prior Art
Natural and synthetic pitches, as is well known, are complex mixtures of organic compounds which, except for certain rare paraffinic-base pitches derived from certain petroleums, such as Pennsylvania crude, are made up essentially of fused ring aromatic hydrocarbons and are, therefore, said to have an aromatic base. Since the molecules which make up these organic compounds are comparatively small (average molecular weight not more than a few hundred) and interact only weakly with one another, such pitches are isotropic in nature. On heating these pitches under quiescent conditions at a temperature of about 350° C.-450° C., however, either at constant temperature or with gradually increasing temperature, small liquid spheres begin to appear in the pitch, which gradually increase in size as heating is continued. When examined by electron diffraction and polarized light techniques, these spheres are shown to consist of layers of oriented molecules aligned in the same direction. As these spheres continue to grow in size as heating is continued, they come in contact with one another and gradually coalesce with each other to produce larger masses of aligned layers. As coalescence continues, domains of aligned molecules much larger than those of the original spheres are formed. These domains come together to form a bulk mesophase wherein the transition from one oriented domain to another sometimes occurs smoothly and continuously through gradually curving lamellae and sometimes through more sharply curving lamellae. The differences in orientation between the domains create a complex array of polarized light extinction contours in the bulk mesophase corresponding to various types of linear discontinuity in molecular alignment. The ultimate size of the oriented domains produced is dependent upon the viscosity, and the rate of increase of the viscosity, of the mesophase from which they are formed, which, in turn are dependent upon the particular pitch and the heating rate. In certain pitches, domains having sizes in excess of two hundred microns up to several hundred microns are produced. In other pitches, the viscosity of the mesophase is such that only limited coalescence and structural rearrangement of layers occur so that the ultimate domain size does not exceed one hundred microns.
The highly oriented, optically anisotropic material produced by treating pitches in this manner has been given the term “mesophase”, and pitches containing such material are known as “mesophase pitches”. Such pitches, when heated above their softening points, are mixtures of two immiscible liquids, one the optically anisotropic, oriented mesophase portion, and the other the isotropic non-mesophase portion. The term “mesophase” is derived from the Greek “mesos” or “intermediate” and indicates the pseudo-crystalline nature of this highly-oriented, optically anisotropic material.
The highly oriented mesophase spheres which begin to appear in a pitch when it is gradually heated are not only optically anisotropic, but also diamagnetically anisotropic, i.e., they have a large diamagnetic susceptibility in a direction normal to the layers of oriented molecules, and a small susceptibility in a direction parallel to these layers. As a result, when pitch containing such spheres is subjected to a magnetic field, the spheres tend to align themselves with their layer planes parallel to the direction of the magnetic field. However, while this orienting effect causes an alignment of the layer planes of the spheres in a direction parallel to that of the magnetic field, the polar or c-axes of the spheres remain free to rotate in a plane perpendicular to the direction of the magnetic field, so that there is no parallel alignment of the polar axes of the spheres.
In accordance with the Singer U.S. Pat. No. 3,991,170, the details of which are incorporated herein by reference, it has been shown that mesophase pitches wherein the layer planes of the mesophase portions of such pitches are substantially aligned in a single parallel direction, and the c-axes of said planes are substantially aligned in a single parallel direction, can be produced by subjecting a mesophase pitch in its molten state to rotational motion relative to a surrounding magnetic field about an axis perpendicular to the direction of that field. The magnetic field subjects the mesophase portions of the pitch to a diamagnetic force which tends to align the layer planes of said mesophase portions in a direction parallel to that of the magnetic field, and when the pitch is simultaneously rotated relative to the field about an axis perpendicular to the field, this diamagnetic force also acts to align the c-axes of said layer planes parallel to the axis of rotation. This unique orientation can be obtained by continuously spinning the pitch in the magnetic field, or rotating the field about the pitch.
The Singer patent also teaches that solid pitch articles can be produced when the planes of the mesophase portions of the pitch are substantially aligned in a single parallel direction, and the c-axes of said planes are substantially aligned in a single parallel direction, thus producing a pitch article which has a preferred plane of increased thermal and electrical conductivity over and beyond that achieved by thermal processes alone.
Further development of the Singer process has been shown in Singer, “Anisotropy of the Thermal Expansion of a Highly-Oriented Mesophase Pitch”, presented at the 19th Biennial Conference on Carbon, at the Pennsylvania State University, Jun. 25-30 (1989), the details of which are incorporated herein by reference.
The prior art also includes a number of processes for producing carbon foams, which have properties of low density coupled with relatively high thermal and electrical conductivity. These foams have their thermal and electrical conductivity substantially identical in any direction. At least two processes have been proposed for the production of such carbon foams with high thermal conductivity. These foams have typically been prepared from so called mesophase pitch. A first technique for production of such foams has involved the injection or saturation of the pitch with a blowing agent, followed by a subsequent drop in pressure to flash the blowing agent and thus foam the pitch. A second process developed at Oak Ridge National Laboratory, has eliminated the use of the blowing agent and instead has heated the pitch under pressure to a temperature sufficient to cause gasses to evolve directly from the pitch and foam the pitch. Both such processes typically start with solid pitch material which has been pulverized into a granular or powder form.
The first type such process, involving the injection of blowing agents and subsequent flashing of the pitch is shown in U.S. Pat. No. 5,868,974 to Kearns, the details of which are incorporated herein by reference. The Kearns process produces a carbon pitch by the steps of:                (a) pressing a quantity of a pitch to provide a pressed article;        (b) placing the pressed article in a pressure vessel;        (c) introducing an inert gas into the pressure vessel under an elevated pressure of about 200-500 psi;        (d) heating the pressed article within the pressure vessel to about 10° to 40° C. above the melting temperature of the pitch;        (e) introducing additional inert gas, under pressure, to obtain a final pressure within the pressure vessel of about 1,000 to 1,500 psi;        (f) holding the pressure vessel and the compressed article under pressure for about 10 to 40 minutes;        (g) venting the pressure vessel to atmospheric pressure, thereby providing a porous foam;        (h) stabilizing the porous foam at an elevated temperature in an oxygen containing environment; and        (i) cooling the resulting, stabilized porous foam to ambient temperature at a cooling rate of about 0.1° to about 0.5° C. per minute.        
The porous pitch foam can be converted to a porous carbon foam by heating the pitch foam in an inert atmosphere to a temperature sufficient to carbonize the pitch. The porous carbon foam can be converted to a porous graphitic foam by heating the carbon foam in an inert atmosphere to a temperature sufficient to graphitize the carbon foam.
The second such process which eliminates the injection of the inert gas or blowing agent, is exemplified by U.S. Pat. No. 6,033,506 to Klett, the details of which are incorporated herein by reference. By the Klett process a carbon foam can be produced by following the steps of:                (a) selecting an appropriate mold shape;        (b) introducing pitch to an appropriate level in a mold;        (c) purging air from the mold;        (d) heating the pitch to a temperature sufficient to coalesce the pitch into a liquid;        (e) applying an inert fluid at a static pressure up to about 1,000 psi;        (f) heating the pitch to a temperature sufficient to cause gasses to evolve and foam the pitch;        (g) heating the pitch to a temperature sufficient to coke the pitch; and        (h) cooling the foam to room temperature with a simultaneous release of pressure to produce a carbon foam.        
The carbon foam so produced can be converted to a graphitic foam by heating the carbon foam article to a temperature sufficient to graphitize the carbon foam.
The carbon foams and/or graphitic foams produced by either the Kearns or Klett processes are generally isotropic in that their thermal and electrical conductivities do not vary dependent upon direction within the foam article.
Additional details on developments related to the Klett type process are found in U.S. Pat. Nos. 6,037,032; 6,261,485; 6,287,375; 6,344,159; 6,387,343; 6,398,994; 6,399,149; 6,430,935; 6,656,443; 6,663,842; 6,673,328; 6,763,671; and 6,780,505; the details of all of which are incorporated herein by reference. It should be noted that foaming using the methods of Kearns or Klett can result in foams with some directionality of properties in the with-rise direction, primarily because of mechanical shearing during bubble formation, although the directionality achieved by Kearns or Klett is insufficient for many purposes.